Tarnish resistant plating for silver articles

ABSTRACT

Articles made of silver or silver alloys containing a high percentage of silver, and particularly silver medals and medallions may be rendered tarnish resistant while still possessing a color closely matched to the original article by plating with a rhodium-platinum alloy. The alloy plating has a thickness of 1 to 50 microinches, and preferably 1 to 10 microinches, and comprises about 1 to 9 weight percent platinum, and preferably 3 to 5 weight percent platinum, with the remainder being rhodium. It is also necessary to provide the silver article with a flash plating of yellow gold prior to plating with the rhodium-platinum alloy in order to obtain the optimum color match.

United States Patent [1 1 Karawin TARNISH RESISTANT PLATING FOR SILVER ARTICLES [75] Inventor: Kenneth J. Karawin, Chester Twsp.,

[73] Assignee: Franklin Mint Corporation, Franklin Center, Pa.

[22] Filed: Mar. 30, 1971 21 Appl. No.: 129,595

[52] US. Cl. 29/119, 75/172 E, 204/40, 204/43 [51] Int. Cl B32b 15/00 [58] Field of Search 29/199, 194; 204/40, 204/43, 47; 75/172 E 5 6] References Cited UNlTED STATES PATENTS 1,991,995 2/1935 Wise 204/47 2,027,358 1/1936 Powell et a1. 204/47 3,515,651 6/1970 Ohkubo et al. 204/43 R 1,947,180 2/1934 Bart 29/199 X 2,364,674 12/1944 Swift 29/199 X FOREIGN PATENTS OR APPLICATIONS 485,905 5/1938 Great Britain 204/43 R [451 July 31, 1973 1,003,848 9/1965 Great Britain 204/43 R OTHER PuaucArrous E. M. Wise, The Platinum Metals, Platinum Metals and Applied Physics Section, The international Nickel Company, Inc., 1953, p. 321-323.

Primary Examiner L. Dewayne Rutledge AttorneySeidel, Gonda & Goldhamme r [57] ABSTRACT Articles made of silver or silver alloys containing a high percentage of silver, and particularly silver medals and medallions may be rendered tarnish resistant while still possessing a color closely matched to the original article by plating with a rhodium-platinum alloy. The alloy plating has a thickness of l to 50 microinches, and preferably 1 to 10 microinches, and comprises about 1 to 9 weight percent platinum, and preferably 3 to 5 weight percent platinum, with the remainder being rhodium. It is also necessary to provide the silver article with a flash plating of yellow gold prior to plating with the rhodium-platinum alloy in order to obtain the optimum color match.

7 Claims, No Drawings TARNISH RESISTANT PLATING FOR SILVER ARTICLES The present invention relates to tarnish resistant silver articles and, more particularly, to tarnish resistant silver base medals and medallions containing about 80 to 100 weight percent of silver.

Silver and silver alloys, particularly fine silver and sterling silver, have long been highly preferred for medals, medallions, jewelry, tableware and various objects of art. This preference is due to the high reflectivity and elegant appearancev which make silver particularly attractive. Unfortunately, a major disadvantage of sterling silver and other silver alloys has been the relative ease with which the silver articles tarnish.

The problem of tarnishing of silver objects has become increasingly severe in recent years. The high concentration of air pollutants has resulted in more rapid and more extreme tarnishing. Particularly harmful tarnishing agents include cigarette smoke and hydrogen sulfide (H 8) which is increasingly present in the atmosphere as a product of fuel oil combustion and various industrial processes.

Much work has been done to develop a nontarnishing silver alloy but to the best of my knowledge no such alloy appears to exist at the present time. Also, various protective coatings and platings have been used to inhibit tarnish, but these attempts have been largely unsuccessful due to lack of commercial acceptance.

The major reason for the lack of widespread use of the various protective coatings appears to be due to the lack of durability. In respect to platings, the lack of commercial acceptance appears to be due to distortion of the characteristic color and high reflectivity which have made silver particularly desirable. For example, protective coatings of pure rhodium have been known for some time to provide excellent tarnish resistance. However, pure rhodium coatings have a cold bluish cast which does not satisfactorily match the silver color of the base material. Such matching of color is extremely important to collectors of silver medals and to owners of other articles made of silver alloys.

Accordingly, it is an object of the present invention to provide a satisfactory tarnish resistant plating for silver articles.

It is a further object of the present invention to provide tarnish resistant silver articles having a protective plating which substantially matches the color and appearance of the uncoated silver base.

It is another object of the present invention to provide tarnish resistant silver medals which are particularly resistant to hydrogen sulfide and cigarette smoke v and still possess the characteristic silver color and appearance. 1

Other objects will appear hereinafter.

The above and other objects of the present invention are achieved by providing silver base medals and other silver articles with a tarnish resistant plating having a thickness of about 1 to 50 microinches and consisting of a metal alloy of about 91 to 99 weight percent rhodium and about I to 9 weight percent platinum. The silver base medal is also provided with a thin layer of yellow gold underlying the rhodium-platinum alloy. It is to be understood that whereever the terms silver base medal or silver article are used throughout this specification, medals or articles containing from about 80 to 100 weight percent of silver are intended.

Although the present invention is in'te'ndedto cover any silver article containing about to 100 weight percent silver, the invention will be described with particular reference to silver medals. The silver base medals to be plated contain about 80 to 100 weight percent silver, and preferably to weight percent silver, the remainder being any of a number of alloying metals. The range of 90 to 100 weight percent silver encompasses both sterling silver (92.5 percent silver) and fine silver as well as other silver alloys. The remaining percentage is usually copper, which is alloyed with the silver. However, other alloying metals may be used.

The tarnish resistant plating which is applied to the silver base medals comprises a metal alloy of about 91 to 99 weight percent rhodium and about 1 to 9 weight percent platinum. A preferred alloy plating contains about 95 to 97 weight percent rhodium and about 3 to 5 weight percent platinum. It should be understood that the term alloy is intended to describe a codeposition of the rhodium and platinum, as opposed to separate depositions or platings of the two metals. Thus, a solidsolid solution of platinum in rhodium is obtained rather than two separate layers.

The codeposition or alloy plating may have a thickness of between about 1 and 50 microinches, and preferably l and 10 microinches. A parituclarly preferred codeposition has a thickness of about 2 to 3 microinches.

While the above described rhodium-platinum alloy platings alone provide excellent tarnish resistance, it is necessary in order to achieve completely satisfactory color matching that the silver base medals first be provided with a yellow gold (pure gold) strike or flash plating. The yellow gold flash plating should have a thickness of about I to 30 microinches, and preferably about 1 to 7 microinches. A yellow gold strike or flash plating followed by a rhodium-platinum codeposition of about 2 to 3 microinches provides a tarnish resistant plating resulting in a particularly good color match for sterling silver medals.

The rhodium-platinum codeposition or alloy platingmay be produced by any one of several well-known methods, including vacuum deposition or electrodeposition. A suitable aqueous electro-plating bath for the codepositions of the present invention comprises an acid bath containing the sulfate salts of rhodium and platinum as described below.

Before the silver base medals are plated with the rhodium-platinum alloy, the medals may be cleaned in a conventional manner, such as by electrocleaning, dipping in various cyanide dips, and/or treating with surface activators. After cleaning, a yellow-gold strike is applied according to well-known procedures such as for one minute at 6 amperes per square foot of afea. The gold strike provides a good seal on the silver base medal for subsequent plating with the rhodiumplatinum bath.

While almost any conventional electroplating apparatus may be adapted to produce the plating of the present invention, a particularly suitable apparatus for achieving both the gold strike and the rhodiumplatinum codeposition is as follows: a conventional electroplating tank is provided with platinized titanium anodes, and the medals are used as cathodes. The medals are suitably held in three fingered stainless steel racks, the fingers of the racks being spaced around the circumference of the medals approximately apart.

The racks are located about 4 to 5 inches from the anodes. The medals should be rinsed according to the usual practice between each of the cleaning and plating steps.

A suitable electrolytic bath for the rhodium-platinum codeposition comprises an aqueous acidified bath of rhodium sulfate and platinum sulfate. The two sulfate compounds should be present in approximately the same relative proportions as the desired relative proportions in the final alloy plating. However, since the relative proportions of each metal codeposited on the silver medal will depend somewhat on the operating temperature and total concentration of the bath, the relative concentrations of each of the sulfate compounds in the bath may be adjusted to the desired operating temperature and concentration, as will be obvious to one of ordinary skill in the art.

The electrolytic bath may suitably be operated at a total concentration of about 0.5 to grams per liter of the sulfate compounds and an operating temperature of about 150 to 200 F, and preferably l65 to 185 F. The bath should also be maintained at an acid pH, and preferably a pH of about 1 to 3.5. The bath may suitably be acidified with sulfuric acid or sulfamic acid.

The electrolytic bath may be operated at a current of between about 5 and 40 amperes per square foot of area for a time of about one-half to 5 minutes or more, depending upon the desired plating thickness. Currents above 40 amperes per square foot of area may produce burning of the plating and currents below about 5 amperes per square foot will be extremely inefficient and uneconomical from a time standpoint. A preferred operating current is about 20 amperes per square foot for about 1 to 2 minutes. The time will of course also be somewhat dependent upon the concentration of the metals in the electrolytic bath.

The electrolytic bath may also contain other wellknown components, such as buffers and cadmium or cobalt brighteners.

After the codeposition has been completed, the silver medals are rinsed and dried according to conventional procedures as desired. For example, to obtain a particularly bright, clean silver appearance, the medals may be rinsed in hot deionized water and then dried.

The plating of the present invention will now be illus trated with reference to the following specific, nonlimiting example:

Sterling silver medals having a diameter of approximately 1 inches and a thickness of approximately one-eighth inch and comprising 92.5 percent silver and approximately 7.5 percent copper are electro-cleaned and then provided with a yellow gold strike according to the above procedure at a current of 6 amperes per square foot for one minute. After rinsing the gold plated sterling silver medals are subjected to a rhodium-platinum electrolytic bath in an apparatus similar to that described above. The bath is operated at a total concentration of approximately 5 grams per liter of the rhodium and platinum sulfates at a temperature of approximately l80 F. The rhodium and platinum sulfates are present in relative concentrations such that the relative proportions of the two metals are approximately 3 to 5 weight percent platinum and 95 to 97 weight percent rhodium. The codeposition is operated at a current of 20 amperes per square foot for approximately 1 minute.

Silver medals plated as in the above procedures have been subjected to accelerated tarnish tests using high concentrations of hydrogen sulfide and other common atmospheric pollutants. In addition, tests have been conducted by exposure of the thus plated silver medals to atmospheric conditions. The tarnish resistance exhibited by the medals plated with the rhodiumplatinum alloy as above compares favorably with the tarnish resistance of silver medals plated with pure rhodium or coated with various lacquers or other resinous substances. More important, however, is the fact that the rhodium-platinum alloy platings of the present invention with the gold strike under-plating exhibit a visual appearance which is substantially identical to an untarnished, unplated sterling silver medal. To the best of my knowledge, such a duplication of the silver color and appearance has heretofore not been possible.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

1 claim:

I. A tarnish resistant silver article comprising a silver base containing from about to 100 weight percent of silver, a flash layer of yellow gold overlying said silver base, and a tarnish resistant plating overlying said flash layer and having a thickness of from about 1 to 50 microinches, said plating comprising a metal alloy layer of about 91 to 99 weight percent rhodium and about 1 to 9 weight percent platinum, the outward visual appearance of said plating being substantially identical to the visual appearance of the unplated silver base.

2. A tarnish resistant silver article in accordance with claim 1 wherein said base is a silver medal.

3. A tarnish resistant silver article in accordance with claim 2, wherein said flash layer has a thickness of from about 1 to 30 microinches.

4. A tarnish resistant silver article in accordance with claim 3 wherein said alloy layer has a thickness of about i to 10 microinches and said flash layer has a thickness of about 1 to 7 microinches.

5. A tarnish resistant silver article in accordance with claim 4 wherein said alloy layer comprises about 95 to 97 weight percent rhodium and about 3 to 5 weight percent platinum.

6. A tarnish resistant article in accordance with claim 5 wherein said silver base is sterling silver and said plating has an outward visual appearance substantially identical to sterling silver.

7. A tarnish resistant silver article in accordance with claim 6 wherein said alloy layer and said flash layer each have a thickness of about 2 to 3 microinches.

i i i i i 

2. A tarnish resistant silver article in accordance with claim 1 wherein said base is a silver medal.
 3. A tarnish resistant silver article in accordance with claim 2, wherein said flash layer has a thickness of from about 1 to 30 microinches.
 4. A tarnish resistant silver article in accordance with claim 3 wherein said alloy layer has a thickness of about 1 to 10 microinches and said flash layer has a thickness of about 1 to 7 microinches.
 5. A tarnish resistant silver article in accordance with claim 4 wherein said alloy layer comprises about 95 to 97 weight percent rhodium and about 3 to 5 weight percent platinum.
 6. A tarnish resistant article in accordance with claim 5 wherein said silver base is sterling silver and said plating has an outward visual appearance substantially identical to sterling silver.
 7. A tarnish resistant silver article in accordance with claim 6 wherein said alloy layer and said flash layer each have a thickness of about 2 to 3 microinches. 